1. Field of the Invention
The present invention relates to an optical scanning apparatus for use in an image forming apparatus.
2. Description of the Related Art
To scan with a light beam onto a photosensitive element, various optical scanning apparatuses are used in image forming apparatuses, such as, a digital photocopier, a facsimile, and a laser printer. According to an optical scanning apparatus that has been conventionally used, a polygon mirror or a galvanometer mirror has been used as a deflector that deflects a light beam from a light source.
However, to form an image in a higher resolution for a shorter time, an optical scanning apparatus needs to rotate a polygon mirror or galvanometer mirror at a higher speed. There is a limitation to rotating the polygon mirror or the galvanometer mirror at a higher speed due to obstacles, such as noise, heat during rotation, and endurance of the bearing that rotatably supports the polygon mirror or the galvanometer mirror.
For this reason, as a deflector used in the optical scanning apparatus, a deflector produced by silicon micro machining is recently proposed (for example, see Japanese Patent Publication No. 2924200, Japanese Patent Publication No. 3011144, and Japanese Patent Application Laid-open No. 2002-82303).
As shown in FIG. 20, a deflector 501 of this type has an integrally molded structure formed of an oscillating mirror 502 and twist beams 503. The surface of the oscillating mirror 502 forms a reflection surface 502a, and the twist beams 503 support the oscillating mirror 502 as a pivot. The deflector 501 has advantages that a small size can be achieved by making the oscillating mirror 502 small in size, and that the deflector 501 works with a low noise and at a low power consumption in spite of that high speed operation is available, because the oscillating mirror 502 is reciprocated and oscillated by using resonance of the oscillating mirror 502.
Furthermore, the deflector 501 has another advantage, which is that, because the deflector 501 causes low oscillation and almost no heat, a housing to accommodate the optical scanning apparatus can be made of thin walls. Thus, the housing is constructed with a resin molding material, at low cost, that contains glass fiber at a low mix proportion, and the image quality is hardly influenced.
Particularly, Japanese Patent Application Laid-Open No. 2002-82303 discloses an example that the deflector 501 is used instead of a polygon mirror. The example proposed is an image forming apparatus that is suitable for an office environment and appropriate to the global environment because low noise and low power consumption are achieved by using an oscillating mirror as a substitute for a polygon mirror.
However, when the oscillating mirror 502 is driven, dynamic surface deformation, due to a moment of inertia and a restoring force of the oscillating mirror 502, occurs as described below.
Suppose dimensions of the oscillating mirror 502 shown in FIG. 20 are 2a in the longitudinal direction, 2b in the transverse direction, and d in thickness; and the density of silicon is ρ. The moment of inertia I of the oscillating mirror 502 is expressed in the following equation 1.Moment of inertia I=(4abρd/3)×a2  (1)
As shown in equation 1, the moment of inertia I of the oscillating mirror 502, which is a local moment, is a function of a distance from the rotation axis to the edge of the oscillating mirror 502, such that, the larger the distance is to the edge of the oscillating mirror 502 from the rotation axis, the larger the moment of inertia will be.
The thickness of the oscillating mirror 502 is a few hundred micrometers, which is thin. Thus, when there is a change in the rotation speed, due to reciprocating oscillation and the moment of inertia applied on the oscillating mirror 502, a force is exerted in opposite directions at both a point in the vicinity of the twist beam 503 of the oscillating mirror 502 and an end away from the twist beam 503. Consequently, the oscillation mirror 502 is waved and deformed as shown in FIG. 21.
Accordingly, a wavefront aberration of the light flux of the light beam reflected by the oscillating mirror 502 becomes large, so that the light beam widens problematically.
FIG. 21 depicts a state of deformation of the oscillating mirror 502 formed as a simple plate. Along with degradation of the wavefront aberration of the light flux, as indicated by dashed lines shown in FIG. 21, deviations of incidence positions are produced in the direction orthogonal to the twist beams 503 (main-scanning direction).
In such a case, apparent curvatures are different, so that imaging positions of the light beam are deviated (out of focus). Particularly due to an assembling deviation of the deflector or the light source, when the light beam is irradiated to an edge of the oscillating mirror 502 as shown in FIGS. 22A and 22B, the light beam at an imaging position 505 becomes out of focus.
Consequently, the light beam irradiated to the edge of the oscillating mirror 502 becomes a converging light flux in the main-scanning direction (see FIG. 22A), or a diverging light flux (see FIG. 22B). As a result, the light beam cannot be uniformly converged onto the imaging position 505, and a desired beam-spot diameter cannot be obtained.
For this reason, conventionally the light beam cannot be converged across an entire scanned surface, and the beam-spot diameter cannot be kept constant, resulting in a problem of degradation of the image.